1. Field of the Invention
The present invention relates generally to self-healing communications networks, and more specifically to a ring topology network which supports multiplex signals on protection communication paths during failures by fast switching from working communication paths.
2. Description of the Related Art
Ring topology networks, particularly, optical ring networks are currently receiving attention because of the number of wavelengths that can be multiplexed onto a single optical link is increasing due to recent innovative techniques. A number of technical publications deal with this topic. A four-fiber ring network is discussed in a technical paper “Multi wavelength Survivable Ring Network Architectures”, A. F. Elrefaie, Proceedings of ICC'93, pages 1245-1251, 1993. According to this publication, a loopback fault recovery method is described. In a four-fiber ring network where optical links are interconnected by a number of network nodes so that working rings are formed for transmission of signals in opposite directions of the ring topology and protection rings are formed for transmission of signals in opposite directions of the ring topology. The protection rings respectively correspond to the working rings and the direction of transmission of each protection ring is also opposite to the direction of transmission of the corresponding working ring. Optical paths are established on each of the working and protection rings between network nodes. If a working optical path between source and destination nodes fails, two loopback points are formed, one on each end of the affected link of the working path, for connecting ends of the corresponding protection optical path to unaffected sections of the working path so that a recovery route is established between the source and destination nodes.
Since the loopback points are close to the location of the failure, the recovery route can be quickly established by nodes adjacent to the fault location and there is no need to exchange fault recovery messages between nodes involved. However, the length of the recovery route is significantly long. If a working path spans across one half of its ring, the length of the recovery route would become one and half times the whole length of the ring.
A two fiber ring network is described in a technical paper “An Optical FDM-Based Self-Healing Ring Network Employing Arrayed Waveguide Grating Filters and EDFA's with Level Equalizers”, Hiromu Toba et al., IEEE Journal on Selected Areas in Communications, Vol. 14. No. 5, pages 800-813. In the two fiber ring network, one of the two rings is used as a working ring for transmission of signals in one direction of the ring topology and the other for transmission of the same signals in the opposite direction. A working path is established on the working ring between two nodes and a corresponding protection path is established between them on the protection ring. Under normal conditions, signals from the source node are forwarded onto the working path as well as onto the protection path. If the working path fails, instant switching occurs at these two nodes to continue the communication over the protection path.
Although all signals can be fully and quickly recovered on the protection path, the constant use of the protection path, utilization efficiency of the transmission mediums is low.